Ion source and accelerator assembly for a time-of-flight mass spectrometer



June 25, 1968 R. s. GOHLKE ET AL 3,390,264

ION SOURCE AND ACCELERATOR ASSEMBLY FOR A TIME-OF-FLIGHT MASSSPECTROMETER 5 Sheets-Sheet 2 Filed Dec. 4, 1964 INVENTORS. Ro/ana 5.oh/A'e Frank/in J. Kar/e Fq. 10 BY A13 1 June 25, 1968 R. s. GOHLKE ETAL 3,390,264

ION SOURCE AND ACCELERATOR ASSEMBLY FOR A TIME-OF-FLIGHT MASSSPECTROMETER 5 Sheets-Sheet Filed Dec. 4, 1964 WQN QWN NWN QQN finUnited States Patent 3,390,264 ION SOURCE AND ACCELERATOR ASSEMBLY FOR ATIME-OF-FLIGHT MASS SPECTROMETER Roland S. Gohlke, Ashland, and Franklin.I. Karle, Natick,

Mass., assignors to The Dow Chemical Company, Midland, Mich., acorporation of Delaware Filed Dec. 4, 1964, Ser. No. 415,897 9 Claims.(Cl. 25041.9)

ABSTRACT OF THE DISCLOSURE This invention relates to electrostatictime-of-flight mass spectrometer apparatus in which a ribbon ofelectrons is brought to sharp focus along the longitudinal axis of theion source, ion acceleration and flight tube assembly. The ions producedby the collision of the electron beam with the sample in the ion sourceare accelerated by means of substantially uniform accelerating fieldsthrough collimating slits in the ion source, ion accelerator, and flighttube and impinge on a detector which usually is an electron multiplierdevice.

This invention relates to an improved mass analyzer and particularly toan electrostatic time-of-flight mass spectrometer having improvedsensitivity and resolution.

Time of flight mass spectrometers of the prior art types have sufferedfrom one or more of the following problems: they have been veryexpensive; have been bulky and not especially adaptable for quickchanges in the type of analytical work in which they were used; had lessresolution than was desirable for many proposed uses; were lesssensitive than was desired, or took too much down time whenever repairsor modifications were made in connection with the instrument.

Accordingly, a principal object of the present invention is to providean improved time-of-flight mass spectrometer.

Another object of this invention is to provide an improved, moreeconomical to manufacture, time-of-flight mass spectrometer.

A further object of this invention is to provide an improved, easier tooperate and maintain time-of-flight mass spectrometer.

Yet another object of this invention is to provide a time-of-fiight massspectrometer which has an improved electron gun assembly.

A still further object of this invention is to provide an improved ionsource for use in a time-of-flight mass spectrometer.

An ancillary object of this invention is to provide an improved ionacceleration assembly for use in a time-offlight mass spectrometer.

An additional object of this invention is to provide an improved morecompact time-of-flight mass spectrometer.

Yet another additional object of this invention is to provide atime-of-flight mass spectrometer having improved resolution.

A subordinate object of this invention is to provide an improved methodof electronically extracting ions from an ion source.

In accordance with this invention, there is provided electrostatictime-of-flight mass spectrometer apparatus in which a ribbon ofelectrons is brought to sharp focus along the longitudinal axis of theion source, ion acceleration and flight tube assembly. The ions producedby the collision of the electron beam with the sample in the ion sourceare accelerated by means of substantially uniform accelerating fieldsthrough collimating slits in the ion source, ion accelerator, and flighttube and impinge on a detector which usually is an electron multiplierdevice.

3,390,264 Patented June 25, 1968 The invention, as well as additionalobjects and advantages thereof, will best be understood in connectionwith the accompanying drawings, in which:

FIGURE 1 is a side elevational and block diagram view of massspectrometry apparatus in accordance with this invention;

FIGURE 1A is an end elevational view of the spectrometer tube shown inFIGURE 1;

FIGURE 2 is an end elevational view, partly in section, of an electronsource in accordance with this invention;

FIGURE- 3 is a side elevational view of the electron source shown inFIGURE 2;

FIGURE 4 is a plan view of the electron source shown in FIGURE 2;

FIGURE 5 is a side elevational view, partly in section, of an ion sourceand flight tube assembly in accordance with this invention;

FIGURE 6 is an enlarged fragmentary cross-sectional view of a tubularelement of the ion source, showing resistive coating on its wall surfaceand an electrically conductive end surface coating;

FIGURE 7 is a sectional view taken along the line 7-7 of FIGURE 5;

FIGURE 8 is a sectional view taken along the line 8-8 of FIGURE 5;

FIGURE 9 is a sectional view taken along the line 9-9 of FIGURE 5;

FIGURE 10 is a sectional view taken along the line 10-10 of FIGURE 5;and

FIGURE 11 is a sectional view taken along the line 11-11 of FIGURE 9.

Referring to FIGURE 1 and FIGURE 1A, there is shown mass spectrometerapparatus 10 in accordance with this invention which comprises anevacuated housing 12 composed of a plurality of sections which asillustrated, an input end section 14 which contains an electron sourceand ion source, a body section 16 which contains a flight tube, and anoutput end section 18 which contains a detector. Electrical and vacuumsystem connections to the various parts of the apparatus are madethrough headers in the various flanges 20, 22, 24, 26 for example.

A vacuum system 28, for example, is coupled to the housing 12 throughthe flange 24.

A power supply 36 is coupled to electron source and ion sourceelectronic circuitry 30 through the cable 32. A clock generator 34 iscoupled to the power supply 36 by means of the cable 38, to the electronsource and ion source electronic circuitry 30 through the cable 40 andto the readout device 42, which may be an oscilloscope or chartrecorder, for example, through the cable 44.

The detector is coupled through the header in the flange 26 and thecable 46 to the readout device 42.

The power supply is coupled, via the cables B, C, and D, to the electronsource (header in flange 22), the detector (header in flange 26), andthe ion source (header in flange 20) respectively.

Referring now to FIGURES 2, 3 and 4 it may be seen that the electron gunassembly of this invention, indicated generally by the numeral 50,comprises a block-like body member 52 having a generally rectangularconfiguration except near one end 54 which is rounded oil to besemicircular.

The member 50 has a pair of outwardly extending flanges 56, 58 at itslower or non-rounded end 60. The member 50 is coupled to a suitable stemassembly 62 which is part of the flange 22 and is adapted to be vacuumsealed to the housing part 14 of the mass spectrometer 10 by means ofthe screws 64, 66, for example. The stem 62, which usually (but notnecessarily) is made of metal, has a plurality of pin connector elements68 extending therefrom on the side of the stem which faces the exterior3 of the mass spectrometer housing. The pin connector elements 68 are,if the stem is made of an electrically conductive material, insulatedtherefrom and from one an other.

A bore 70 is disposed adjacent to the rounded end 54 of the body member52, extending completely through the member 52. The bore 70 isperpendicular to the wall 72 and in axial alignment with the outersurface 74 of the member 52.

The bore 70 has a counter-bore 75, 76 at each end.

An annulus 78, 80 is provided which is made of an electricallyinsulating material of good thermal conductivity, such as syntheticruby, for example, and has an outer diameter such that one of the annulimay be press-fitted into each of the counter-bores 75, 76. A slot 82,usually having parallel sides, extends between the inner diameter andouter diameter of each annulus 78 or 80. The width of the slot 82 isequal to or greater than the width of the slot 84 which extends acrossthe top of the round-ed end 54 of the body member 62. The slot 82 andthe slot 84 are axially aligned with respect to the bore 70.

Arotted tubular member 86 having an electrically conductive inner wallsurface and a generally C-shaped transverse cross-sectionalconfiguration is disposed between the annuli 78 and 80, the outerdiam-eter of the tubular member 86 being such with respect to the innerdiameter of the annuli that it may be press-fitted between the annuli.The width of the slot 88 i the member 86, which is a beam focusingelectrode, is less than or equal to the width of the slot 82 in theannuli 78 or 80.

A threaded bore 90, extends through the side wall of the body member 52at or near its rounded end 54.

An electrically insulating bushing 92 engages and extends through thebore 90. An electrical lead 94 extends through the bushing 92 and iselectrically connected to the conductive inner surface of the member 86.Actually, the member 86 is usually made of metal, such as copper, forexample.

Referring especially to FIGURE 4, as well as to FIG- URES 2 and 3, itmay be seen that the rounded end 54 of the body member is flattened overat least a part of its surface so that, at the flattened part, thethickness of the end wall is only a few thousandths of an inch (.002inch is commonly used). The surface 96 of the flattened part of the end54 is substantially parallel with respect to the surface of the end part60 of the body member 52. The length of the flattened surface 96 (asmeasured along the slot 84), is about of the length of the slot 84. Thesurfaces 98, 100, each beginning at an end of the flat surface 96, isbeveled upwardly at an angle of approximately 45 degrees with respect toan endwise extension of the flat surface 96.

A pair of filament mount support flanges 102, 104 extend outwardly fromthe wall surfaces 72, 73 of the body member 52 intermediate of the ends54, 60. The flanges 102, 104 are rigidly coupled to the body member andmay, if desired, be an integral part of the block member 52, as shown.

Each of the flanges 102, 104 has a bore 106, 108 extending therethrough.The axis of each of the bores 106, 108 is parallel with each other andwith the wall surfaces 72, 73. An electrically insulating bushing 110,112 having an internally threaded bore 114, 116 therein is press-fittedinto each of the bores 106, 108 in the flanges 102, 104.

An electron source filament support element 118 or 120 having a threadedend 122 or 124 and a slotted, thinned spring-like end 126 or 128 iscoupled to each of the threaded bores 114, 116, the slotted thinned endsbeing so aligned that the bottom of the slots in the ends 126, 128 isbelow the longitudinal axis of the bore 70 by a distance approximatingone half the diameter of the wirelike electron source (filament) 130.

The filanmntary electron source 130 illustrated is a tungsten wirehaving stop means disposed intermediate of its ends 132, 134. While thestop means may be a knot in the wire, it is often easier, from amechanical construction standpoint, to spot weld a small metal elementto the tungsten wire at an appropriately spaced distance along the wire.The space between the stop means should be such that the spring-likeends of the electron source support elements holds the wire firmly intension.

If the filament 130, the slotted tubular member 86, and the slot 88 inthe rounded end 54 are properly made and aligned, a single plane shouldpass midway between the slot 84 and slot 88, and pass all along thelength of the filament 130, while the filament is equidistant from anypoint on the inner surface of the focusing electrode 86 (measuredperpendicularly).

Referring now to FIGURES 5 to 10, there is shown an ion source assembly,indicated generally by the numeral 150, comprising a first electrodeelement 152 (see also FIGURE 8) which is a disc-like element having adiameter which is several times its thickness and has an annular flange154 disposed on one side thereof concentrically with respect to thecenter of the disc-like electrode element. An array of small diameterbores 156 extend axially through flange 154 of the electrode element152.

A rod-like metal element 160 extends from the center of the side of theelectrode element 152 which is opposite the side having the flange 154therein.

The electrode element 152 and the rod 160 may be an integral structureor the rod 160 may be secured to the electrode 152 as by a fusioncoupling, for example, or other suitable rigid coupling means.

The end 162 of the rod 160 which is remote from the electrode element152 is rigidly coupled, as by a weld, for example, to a disc-like baseelement 164 which has an array of terminal pins 166 extendingtherethrough and insulated therefrom. The pins 166, are, of course,electrically insulated from the base element 164. The base element 164is adapted to be coupled in a gas-tight sealing relationship with thehousing section 14 of the mass spectrometer 10.

A tubular element 168 having a circular transverse cross-sectionalconfiguration and an outer and inner diameter such that its end isclosely but slidably in the flange 154 in the electrode element 152,extends from the grooved side of the element 152. The length of thetubular element 168 is a minor fraction of its diameter. The tubularelement 168 has an electrically conductive pyrolitically depositedcoating 170 on it inner wall surface (see FIGURE 6 for details) and acoating 172 of electrically conductive metallized paint (a silvercompound is commonly used) at its end.

The tubular element 168 has diametrically oppositely disposed slots 174,176 (see FIGURE 9, especially) which lie along a plane parallel with theends of the element 168. The length and width of the slots 174, 176 aresuch that the ribbon electron beam emanating from the electron gun 50may pass therethrough without impinging on the tubular element 168.

As seen in FIGURES 9 and 11 a wire-like electrode 178 is disposedadjacent to but spaced from the outer wall of the tubular element 168 inaxial alignment with the slots 174, 176, and serves as an electron trap.A rigid electrical lead 180 holds the trap electrode 178 in position andis coupled (by means not shown) to one of the pins 166 in the header 24.

A metal annular member 182 is provided which has a circular groove 184or 186 in each side surface. The outer diameter of the member 182 isapproximately the same as the outer diameter of the element 152. Theinner diameter of the member 182 is slightly less than the diameter ofthe tubular element 168. A disc 188 having a slot 190 therein is fixedlycoupled to the member 182 by means of screws 192, the disc 188 spanningthe open inner part of the member 182.

The ends of the tubular element 168 lie in the groove 186 and adjacentto the flange 154 (end 196 in groove 186).

A tubular element 194, like the tubular element 168 except that it hasno slots (as 174, 176, for example) and has greater length, has its end198 fitted into the groove 184 (the end 196 of element 28 is fitted intothe groove 186). The coating 199 on the inner wall of the element 194 isessentially the same as the coating 170 on the section 168, as shown inFIGURES and 6, for example. The elements 168 and 194 may be made ofglass, for example, although other insulating materials may be used.

A metal annular member 200, which is physically identical to the annularmember 182, is coupled to the end of the tubular element 194 which isremote from the memher 182. A disc 202 having a slit 204 therein iscoupled to the member 200, the disc 202 being similar in form to thedisc 188.

A tubular element 205, shorter in length than the length of the tubularelement 194, but otherwise identical in physical form to the element194, has one end fitted into the groove in the annular member 200 whichcorresponds to the groove 184 in the member 182. The element 205 has aconductive coating 206 on its inner wall surface and conductive coatingon its endsurfaces as do the elements 168 and 194.

Referring now to FIGURE 7 as well as to FIGURE 5, it may be seen thatthe structure of the annular member 208 is the same as that of theannular member 182. However, instead of a disc having a slot in it beingcoupled across the open central part of the annulus, an annular shapedinsulating bushing 210 having an L-shaped transverse cross-sectionalconfiguration is coupled to one side of the member 208 by means ofscrews 212 made of insulating material. Deflection plates 214, 216, eachof which is rectangular in configuration, are rigidly connected tosupport plates 218, 220. The deflection plates are generallyperpendicular with respect to the support plates. The support plates aresecured to but insulated from the annular member 208 by being disposedagainst the insulating bushing 210 and held in place by the insulatingscrews 212.

The deflection plates 214, 216, are each displaced an equal distancefrom the longitudinal axis of the annular member 208 and are wider thanthe thickness of the member 208.

A tubular element 222, shorter than the tubular element 205, has an endtelescoped within the groove 223 of member 208. Like with the element168, 194, a conductive coating or surface is on the inner wall of theelement 212.

The other end of the tubular element 222 is telescoped within thegrooved end 224 of a metal annular member 226 whose surface facing theelement 208 corresponds in configuration to the surface of the element200 which faces the element 208. Thus, a disc 226 having a slot 228therein is secured to the element 226, the slot 228 being axiallyaligned with the slots 204 and 190.

A flight tube assembly, indicated generally by the numeral 230, has aflanged base 232 and an elongated tube 234 of metal, such as stainlesssteel or copper, for example. The inner diameter of the tube 234 isapproximately the same as the inner diameter of the annular members 182,200, 208 and 226, for example.

A fine mesh metal screen 236 is disposed across the output end 238 ofthe flight tube 234. A 90 mesh nickel screen has been successfully used.Screens having from 50 lines per inch up to the limit wheretransmission,

through the mesh becomes too small are practical. The fine screenprevents any substantial penetration of the drift tube by externalfields.

Insulating spacer screws 240 made of nylon, for example, are disposed inarray around the periphery of the flight tube 234 intermediate of theends of the tube 234. The length of the flight tube is approximately 40centimeters. The ion source assembly 150 and the flight tube assembly230 are held together to form a unitary structure by means of aplurality of bolts 242 made of insulating material which extend from themember 152 along the peripheral part of the ion source assembly and theperiphery of the base 232 of the flight tube assembly.

The ion source and flight tube assembly is inserted in the housing 12 ofthe spectrometer apparatus by removing the flange 20 to which thedisc-like base element 164 is coupled, and sliding the assemblies intothe housing tube. The flight tube assembly and the ion source assemblyare supported within and insulated from the housing 12 by means of thespacer screws 240 and the disc-like header element 164 which is securedto the flange 20.

The detector used in this apparatus may be any of a number ofconventional detectors used for this purpose, an electron multipliertype of detector being commonly used.

The electron source assembly, coupled to the flange 22 on the housing12, is inserted into the housing perpendicularly with respect to thelongitudinal axis of the ion source assembly. The slot 84 of theelectron source is axially aligned with respect to the slots 174, 176 inthe element 128.

In operation the potential on the electron source focusing electrode 86is adjusted to cause the electron beam emanating from the electronsource to come as nearly as practical to a line focus in axial andplanar alignment with the slots 190, 204, 228 in the ion sourceassembly.

The sample material to be analyzed may be introduced into the spacedefined by the members 152, 182 and element 168, known as the iongeneration chamber, through a suitable port, such as a septum 244 in theflange 246 which is coupled to the housing section 14. The septum 244 isaligned with a small bore 248 in the member 168, and sample may beinserted by means of a hollow needle, small tube, or other means knownto those skilled in the art.

During operation, voltages derived from the electron source and ionsource electronic circuitry are repetitively applied to the block-likebody member 52 (and thus across the slot 84) of the electron source andto the member 182 of the ion source while the remainder of the ionsource is held under a constant accelerating field. l

The manner of applying the repetitive voltages to the ion generatingregion and the remainder of the ion source assembly is shown insimplified form in FIGURE 5 by means of the voltage dividing resistor248 which actually represents the resistance of the resistive coating onthe inner wall surfaces of the tubular elements 168, 194, 205 and 222,for example. For the sake of simplicity, the leads 250, 252, which areconnected to the deflecting plates 214, 216 (by fused connection to themetal parts 218, 220, for example) and which provide some focusing ofthe ions passing from the ion source through the slit 228 are shown asbeing connected to the voltage divider resistor 248 rather than to anexternal voltage source.

When a suitable voltage is applied across the slot 84 of the electronsource which permits the electron beam to pass into the ion source, theswitch 254, which is coupled to the resistor 248 at the junction 256 andto the metal member 182, is opened, providing an electrical field in theion accelerating region which urges ions formed as the electron beamimpinges on the sample in the region to be accelerated towards andthrough the slot 190.

As indicated by the connections 256, and 262 along the voltage divider,the ions are subjected to accelerating fields (which are uniform in eachsection of the ion source because of the conductive resistive coating onthe inner wall surfaces of the elements 194, 205 and 212) beforeentering the drift tube 234 which is at the same potential along itslength as the potential on the member 226.

The purpose of the electrode 178 is to be a trap for electrons whichpass through the ion generation region of the ion source from theelectron gun (through the slits 174, 176 which are about wide). Theelectrode 178 also provides a convenient means by which electron beamcurrent may be measured.

After each group of ions are urged down the ion source and into theflight tube, they separate in their passage in accordance with theirmass as is well known in the art of time-of-flight mass spectrometry andsuccessively impinge on the detector, are amplified, and are displayedon a readout device on a time base and amplitude of received signalscale. The readout device and the application of voltages to theelectron beam and ion source are synchronized by means of the clockgenerator, as is well known in the art.

In general, materials used in the apparatus of the invention should notout-gas and must be non-magnetic.

In one apparatus made in accordance with this invention, the operatingpotential on the member 152 is ground, the potential on the member 182is between 50 and 300 volts, the potential on the member 200 is about-3,000 volts, the potential on the plate 214 is about 3,000 volts, thepotential on the plate 216 is about 3,000 volts -5%, and the potentialon the member 226- and flight tube 234 is typically 3,000 volts.

The potential of between 50 and 300 volts on the member 182 isdetermined by setting the voltage to give the best resolution for theinstrument.

The detector used may be an electron multiplier tube (with glassenvelope removed) such as an RCA type 7746 or EMI type 9603, forexample.

What is claimed is:

1. Ion source, accelerating and focusing apparatus for use intime-of-flight spectroscopy apparatus, comprising:

(A) an ion source chamber which is generally cylindrically inconfiguration, said chamber having ends made of electrically conductivematerial, one of said ends having a slit therein, and cylindrical sidewalls, said side walls being by an electrically resistive inner surfaceand having diametrically opposed slits extending therethrough, saiddiametrically opposed slits being perpendicular to said slit in the endof said chamber, means for introducing a sample into said ion sourcechamber,

(B) a generally cylindrical ion accelerating chamber having as its inputend the end of said ion source chamber having said slitted end, itsoutput end of electrically conductive material having a slit which is ofthe same dimensions and is axially aligned with its input end slit,cylindrical side walls having an electrically resistive coating coveringtheir inner surface, and

(C) a generally cylindrical ion focusing chamber having as its input endthe slitted output end of said ion accelerating chamber, an output endof electrically conductive material having a slit therein which is ofsimilar dimensions and is axially aligned with the slit in the input endof said focusing chamber, a pair of cylindrical side wall elements, eachof said side walls being covered by an electrically resistive inner wallsurface, said pair of side walls being disposed in end-to-endrelationship and having ion beam focusing support structure sandwichedtherebetween, a pair of ion beam deflecting plates, said deflectingplates being axially aligned with respect to said slits in the input endand output end of said focusing chamber and spaced apart a distancewhich is greater than the width of said last men tioned slits, and

(D) means for sequentially applying a potential difference between theends of said ion source chamber, means for applying .a potentialdifference between the ends of said ion accelerating chamber, means forapplying electrical potentials to the ends of said ion beam focusingchamber, and means for applying electrical potentials to said deflectingplates.

2. Apparatus in accordance with claim 1, wherein the electricallyresistive side wall surfaces of each of said chambers are electricallycoupled to the ends of the respective chambers.

3. Apparatus in accordance with claim 1, wherein said all of said sidewalls are vitreous and have a resistive coating on their inner surface.

4. Apparatus in accordance with claim 1, wherein the inner diameter ofeach of said chambers are substantially the same.

5. Apparatus in accordance with claim 1, wherein the spacing betweensaid deflecting plates is approximately three times the width of theslits in the output end of said focusing chamber.

6. Apparatus in accordance with claim 1, wherein said deflecting platesare disposed parallel to the longitudinal axis of said chamber.

7. Apparatus in accordance with claim 1, wherein said chambers aredisposed within an evacuated housing.

8. Apparatus in accordance with claim 1, wherein said chambers arephysically coupled together as a unitary structure.

9. Apparatus in accordance with claim 1, wherein a stem assemblyincluding electrical lead feed-through elements is coupled to the end ofsaid ion source chamber which has no slit therein.

References Cited UNITED STATES PATENTS 2,612,607 9/1952 Stephans 25041.92,662,184 12/1953 Berry 250-419 3,163,752 12/1964 Wahrahaftig et al.250-419 2,231,676 2/ 1941 Muller 250-207 RALPH G. NILSON, PrimaryExaminer.

ARCHIE 'R. BORCHELT, Examiner.

A. L. BIRCH, Assistant Examiner.

